A steam control valve is used to control the steam flow from a steam generator to a turbine in power plants. The flow field in the steam control valve is usually complex and transonic. A flow-induced vibration can occur in the valve and causes unexpected shutdown. An experimental and numerical study is conducted to clarify the characteristics of the transonic unsteady flow in the steam control valve. The results show the existence of a steady flow pattern and five unsteady patterns depending on opening ratios and pressure ratios. Both the experimental and numerical results indicate that even under the same condition, different flow pattern may appear depending on the process of changing the pressure ratio.
